Heat sensitive circuits



Dec. 12, 1950 o. H. SCHMITT HEAT SENSITIVE CIRCUITS Filed July 22, 1946 TEMPEf/Q TUBE TEMPEQHTUEE 0 770 ll. SCI/M177.

Patented Dec. 12, 1950 This invention" relates to? electrical i circuits: More particularly; itrelatesitothermistors; which: are conductors or semiconductors the resistance-- of which is a function ot temperature: Thermistors are circuit elements havingvery-large temperature coeflicient of resistance and' are described" in a: paper by"Becker:-et -3 ali entitled "Propertiesand-Uses of-Tliermistors+-Thermally Sensitive Resistor-s3" Electrical venience *in' nomenclature the term thermistor as used-*in the instant specification and claims shall be understoodas-meaning devices of the character describedirl the aforesaid publication:

' From its primary propertiesas an element, the

thermistor-ha been usedfiortemperature indicatingand control purposes; and "when intermittjently heated; either externally orinternally; it has "been used-for timmdelay purposes: In its various applications, its resistance hasbeen-taken as decreasing indefinitely as the temperature rises within its operating-- limits:

' It isth'eobjcect of "this-invention to extend the utility of thermistors" and to provide circuits wherebyadd-itionalproperties-may in--effect'be attributed "to them;

his a further *objectmf *this inventiomtoproviclea device responsive tomultiplecritical temperatures or to a critical temperature zone-.

D Other andfu'rther objectsofthednvention are 2 trically isolatedfrom--theremainder of "the circult: This bridge has notable temperature response 'and time de'lay characteristics, aswilt be clear from the detailed disclosure b'elow;

- Furthermore; thisinventiomin effect extends thermistor properties to Y have multiple critical. temperature ranges: cr -a; critical temperature band=over "which" thedevice-is responsive: This is accomplished by arranging one thermistor in series with" a resistor, and in series th'erewith a thermistor and resistorareconnected in parallel;

these two resistor-thermistor units arranged: as adjacentarmsofa bridge; The: unbalancewolt' agewill vary-with heating: of the thermi tors in the bridge arms so i that anothersth'ermistor; in the diagonal of the bridge; will} heavariably Engineering, Transactions;-Nov:1946}pagesflll1 125;- For-con- 2 heatediito yieldia: doublethumprcurveroi g'diagonal thermistor resistance-v plottedzagainsti-temperas ture ofv the'thermistors inth'ebridgei-arms; Re. actorszmay" be:- substitutedz forthe. resistors for alternating; current :service; though" IQSlStOZ'Sl-MQ- preferab1e.= V 1 For; a more 1:completesaunderstandingqoi.ther ine vention; reference is madezetoi-tlre ti'ollowing -de tailed'description and to-the-drawings; in which;

Figures 1 and z'are iliustrativeespeciesrof'z'elem trical circuits embodyingrcertain aspects -otithis inventiom:

Eigure .-3 is aatermieratureeresistancecharac teristicsoi thespecies-in Figura-l: and? Figures 4 4 and? 5 area temperaturesresistance characteristics that may bet-obtained front; the circuit orFigure-Ziusing various constants-s.

Referring now to" Fi'sure.=:1;,. cwer-i supply: I0 whichemay bereither directmurrentr: orrralternating current is impressed on terminals Hand? ll of! an impedance :bridge whileheating element I 6 is connecte d to terminalsswiandifli'oi .theabridge. Impedances: 22 and: '24 F are, connectedi between terminalszl Ztand 20-,- and-ztfiandtill; respectively, while thermistors' 28:1 and; 285 are: connected zbe+ tween terminals l Zandil 8; andzl'andfl; respectively; A thermistor element Qlislocated closely adjacent heater l6. Impedanceszl'and flare preferably'matchedresistors;and elements "and. 2 8 a are: preferably matched thermistors; ,thez. re sistance of eaclrof fwhich is :equal to that otieach resistance 22 and; 24- atiaxpredetermined'temperature.

It 1 will be: evident that the unbalance .of 3 the bridge will depend upon the: temperature to-whicli thermistors 26: and 28 are" botli exposed; in" the area-indicated by dotted circle 32" For tempera+ tures-helow 0rabove that atiwhich 'bal'anceis'ohtai-ned the unbalance-current inwresistorl8 1 will h'eat'thermistor element 3I' 'Sosth'at its resistance will sharply decrease. Only for? a limited ttem perature range with aisharp peakewillithezreslst ance 0f thermistor 'elementiifi' rise; This :rise is sharp" and extends" over; a::. limite d-=' temperature range;- It is notable that the resistancei of a thermistor element should" rise i'or 'anielevated temperature; a characteristic that' -isxadaptable to a' wide variety of uses and iST-COMTQEY UO the usual behavior of thermistorsi The circuit ot- Figure l mayb'emodifl'ed ito diminish the-=sliarpnessof" the-responsecurve :-by su-hstitutine a resistor 'forone-0tthe-thermistors; Where only one thermiston is'used inwthe bridge an alternating current' power supply issused; appropriate-reactorsmay be'?usediin raanywot li arms. Where isolation is unnecessary, a less practical arrangement is to heat thermistor element 30 directly by passing the unbalance current through it.

The peaked characteristic in Figure 3, which .represents the resistance of thermistor element 30 plotted against temperature of the other thermistors, mayjbe simulated in the case of the circuit in Figure 2- for multiple critical temperatures as in Figure and for a temperature zone (with two critical peaks) as in Figure 4. In Fig'- ures 4 and 5, it is the resistance of thermistor element 34 which is plotted against the temperature of the thermistors in the bridge arms.

In Figure 2, power supply 36 is connected to terminals 38 and 44 of the bridge. Heater 42 for thermistor 34 is connected between terminals c4 and 46 of the bridge. Matched thermistors 48 and 50 are connected inseries with matched resistors 52 and 54 between terminals 38 and 44, and terminals 4ii-and: 46; respectively. Matched thermistors 56 and 58 are connected in parallel with matched resistorsfiil and 52 between terminals 4B and 44, and 38 and 46, respectively. Resistors 6B, andBZare preferably alike but may be the same or difierent from resistors52 and 54.- Thermistors '48 and 50 may or may not match thermistors Stand 58. Theyare arranged to be, exposed to the same temperature within dotted circle 64. All the resistors and thermistors may be'dissimilan. I

The curve of Figure 4 was obtained through the use of Bell Telephone Laboratories thermistor type ESP-629867 at 56 and 58, while BTL type ESL-630321 was'used at 48 and 50. Thermistor 34 and its heater were BTL type D-158997. Re sistors 52, 54, 6!! and 52 were all .100 ohms. The curve of Figure '5 was obtained with the same constants exceptthat resistors (ill and E2 were 300 ohms. 1 V It is evident that the circuit of Figure 2 may be used with the thermistors within circle 64 arranged for temperature measurement and for operation'of controls for a critical zone or zones in a chemical process or the like.

The bridges of Figures 1 and 2 may be used for their temperature-response characteristics discussed above, by connecting to thermistor elements 3i! and 34 a resistance-measuring device, calibrated according to the temperature of circles 32 and 64. Those bridges may also be used for time-delay or timing sequence purposes. Thermistors in the past have been heated by the current they pass or by an external resistor, and when so arranged, have been used to operate various single devices in a single manner after a predetermined time interval. With suddenly started heating of the thermistors in circles 32 and 64 optionally by the bridge current but preferably by an auxiliary heater, the resistance of thermistor elements 30 and 34 varies from a high, normal value to a low value, then to a high value .(Figure 3) again and further to a low value, which cycle is substantially repeated in Figures land 5. This reversal, optionally repeated, is an obviously useful effect.

.. For convenience of definition, the impedances 22 and 24 of the bridge circuit shown in Figure 1 are regarded as opposite branches of the bridge circuit; similarly the thermistors 25 and 28 are opposite branches of thebridge circuit. Likewiseinregard to Figure 2, in the bridge circuit there shown, the branch composed of impedance 5'2 and thermistor 48 in series is considered as appo i etc th ihr nah san a ine impedance 4 and thermistor in series. Similarly the branch containing impedance 62 and thermistor 58 in parallel is considered as opposite to the branch containing impedance and thermistor 56 in parallel.

As many apparently widely difierent embodiments of this invention may be made Without departing from the spirit and'scope' thereof, it is to be understood that I do not limit myself to the specific embodiments herein except as defined by the appended claims.

What I claim is:

1. An impedance bridge circuit of the Wheatstone type having input and output terminals and containing first and second thermistor elements, one in each of two opposite branches of the bridgecircuit, .and first and second impedances, one in each'of the other pair of opposite branches "of the bridgecircuit, and means for heating said first and second thermistors simultaneously, said circuit being further characterized in that the first and second impedances each has a thermistor in series therewith and the first and second thermistorseach has an impedance in parallel therewith.

2. An impedance bridge circuit of the Wheatstone type having input and output terminals and containing first and second thermistor elements, one in each of two opposite branches of the bridge circuit, and first andsecond impedances, one in each of the other pair of opposite branches of the bridge circuit, and means for heating said first and second thermistors simultaneously, said circuit being further characterized in that the first and second impedances each has a thermistor in series therewith and the first and second thermistors eachhas an impedance in paral-.- lel therewith, said series thermistors being heated simultaneously with said first and second thermistors.

3.;A heat sensitive thermistor bridge comprising first and second input terminals and first and second output terminals, a first thermistor and first-resistor in series therewith connected between the first input and second output terminals, a second thermistor and second resistor in series therewith connected between the second input and first output terminals, a third thermistor and third resistor in parallel therewith connected between the first input and first output terminals and a fourth thermistor and fourth resistor connected between the second input and second output terminals, and a common enclosure for said thermistors.

4. The apparatus of claim 3 further characterized in that a fifth resistance element is connected across the first and second output terminals and a fifth thermistor is located in heat communication therewith.

5. In a thermistor system having a responsive thermistor, a heating element therefor and a supply circuit therefor, said supply circuit having first and second input terminals and first and second output terminals, a first thermistor and first resistor in series therewith connected between the first input and second output terminals, a second thermistor and second resistor in series therewith connected between the second input and first output terminals, a third thermistor and third resistor in parallel therewith connected between the first input and first output terminals and a fourth thermistor and fourth resistor connected between the second input and second output terminals, and a common enclosure for said thermistors, aid heating element 5 beingconnected to the first and second output terminals of the supply circuit.

O'I' I 'O H. SCHMI'I'I.

REFERENCES CITED The following referencesare of record in the me of :this patent:

UNITED STATES PATENTS Number;

I Name Date 1,694,264 Hull Dec. 4, 1928 2,025,775 Rieber Dec. 31, 1935 2,175,890 Glowatzki Oct. 10, 1939 2,193,578 Bruce Mar. 12, 1940 2,230,243 Haficke Feb. 4, 1941 Number Number 16 144,669

Name Date Bagnall Apr. 7, 1942 Fairley et a] Mar. 7, 1944 Scully May 8, 1945 Pearson Mar. 5, 1946 Lamm Oct. 28, 1947 FOREIGN PATENTS Country Date Great Britain Sept. 9, 1921 OTHER REFERENCES Keystone Carbon (30., February, 1945, Electronics, page 169. 

